In the swage joining of two oil well casings by hydraulic expanding devices, a permanent deformation often results in two overlapping casings, depending on the type of joint formed. Where six hydraulic swaging rams are utilized in joining a smaller upper end of one casing to a larger bottom end of an upper casing, as by dimpling as illustrated in my U.S. Pat. No. 3,555,831, the free upper edge of the smaller inner casing end often distorts into a hexagonal shape with six straight sides spaced from the wall of the larger upper casing. These straight sides accordingly leave less working space in the casings for drill bits, drill stems, etc., for example.